Host controls of within-host disease dynamics: insight from an invertebrate system

Within-host processes (representing the entry, establishment, growth and
development of a parasite inside its host) may play a key role in parasite
transmission but remain challenging to observe and quantify. We develop a
general model for measuring host defenses and within-host disease
dynamics. Our stochastic model breaks the infection process down into the
stages of parasite exposure, entry, and establishment and provides
associated probabilities for a host’s ability to resist infections with
barriers and clear internal infections. We tested our model on Daphnia
dentifera and the parasitic fungus, Metschnikowia bicuspidata, and found
that, when faced with identical levels of parasite exposure, Daphnia
patent (transmitting) infections depended on the strength of internal
clearance. Applying a Gillespie algorithm to the model-estimated
probabilities allowed us to visualize within-host dynamics, within which
signatures of host defense could be clearly observed. We also found that
early within-host stages were the most vulnerable to internal clearance,
suggesting that hosts have a limited window during which recovery can
occur. Our study demonstrates how pairing longitudinal infection data with
a simple model can reveal new insight into within-host dynamics and
mechanisms of host defense. Our model and methodological approach may be a
powerful tool for exploring these properties in understudied host-parasite
interactions.